Mainly, chloroprene has been so far used as rubber materials for use in the conventional CVJ boots, and it is the current status that temperature elevation in the engine room and its surrounding regions due to recent requirements for higher outputs of automobile is making the conventional rubber material thermally unsatisfactory.
Recommended substitute materials includes ethylene-alkyl acrylate copolymer rubber (as will be hereinafter referred to as AEM) as one of heat-resistant polymers. A variety of hardness and low temperature durability are required, depending on product uses. For example, lower hardness and improvement of low temperature durability need such means as addition of a plasticizer, reduction in the amount of carbon black, etc.
However, in the case of AEM, the addition of a plasticizer may bring about air inclusion or lowering of physical properties due to lowering of cross-linking density at the time of peroxide cross-linking, and the reduction in the amount of carbon black is not only may lower physical properties such as rubber strength, etc., but also may give rise to air inclusion at the time of vulcanization molding. A countermeasure against the air inclusion includes an increase in the amount of organic peroxide, which may however result in elevation of hardness, lowering of elongation, and even air inclusion due to gases generated from the organic peroxide itself, so any of these means has been found practically unsatisfactory.
To solve these problems it has been so far proposed to blend AEM with a specific ethylene-propylene-diene terpolymer (as will be hereinafter referred to as EPDM) or oil extended EPDM, followed by peroxide cross-linking to lower the hardness, which is effective for combination with paraffinic plasticizers, but not for combination of oil non-extended EPDM with other plasticizers such as ester-based ones, etc. having a good compatibility with AEM, because of occurrence of such inconveniences as air inclusion, etc. That is, there is such a selectivity as limited plasticizers.
Patent Literature 1: JP-A-2000-143894.
Addition of EPDM has an effect on improvement of low temperature characteristics, but in the case of said Patent Literature 1, cross-linking is carried out only with an organic peroxide, but amine vulcanization may give rise to air inclusion, and thus even the vulcanization system has a limited selectivity.